The present invention concerns apparatus and a method for locating defects in items having regular patterns and in particular to a dark field imaging technique which may be used to inspect semiconductor wafers having fine repetitive patterns.
The detection of particles and defects on patterned wafers is a problem critical to the semiconductor industry. Contaminating particles on the surface of a semiconductor wafer can result in unintended conduction paths in the integrated circuits formed on the wafer. Defects in one or more of the photolithographic patterns which are used to produce the integrated circuits can produce non-functioning or substandard devices. It is important to identify the type and characteristics of any defects in the integrated circuits at various processing stages so that the cause of the defect can be corrected before it can adversely affect yield.
In the prior art, there are many ways to locate defects and contaminating particles on the surface of a semiconductor wafer. Generally, these methods fall into one of three classes: spatially filtered bright field imaging techniques, image analysis of bright field images and low-angle dark field imaging techniques.
One such prior art reference in U.S. Pat. No. 4,771,468 entitled SYSTEM FOR AUTOMATIC INSPECTION OF PERIODIC PATTERNS is characteristic of a bright field inspection technique that recognizes particles and pattern defects through image processing of the bright field image (without spatial filtering). The basis of the algorithm is to compare corresponding picture elements (pixels) from supposedly identical array elements on the circuit. If the center pixel matches the corresponding left and right pixels, there is no defect. Additional processing is used to test for instabilities which may occur when pixels are located next to lines or on rough surfaces, when there is interfering noise from the camera or when there are systematic changes in the optical properties of the parts being inspected. While the information produced by such a system is essentially the same as that produced by the disclosed embodiments of the present invention, an important difference is the system described in the referenced patent is relatively more expensive (because of the electronic image analyzer) and relatively slow (depending on the sophistication of the image analyzer.
Another system is described in U.S. Pat. No. 4,806,774 entitled INSPECTION SYSTEM FOR ARRAY OF MICROCIRCUIT DIES HAVING REDUNDANT CIRCUIT PATTERNS, which is hereby incorporated by reference for its teachings on optics and the inspection of semiconductor wafers. This system uses a Fourier transform lens and an inverse Fourier transform lens positioned along an optical axis.
The system forms a bright-field image of an area on the wafer at a distant image plane. In this system, spatial frequencies corresponding to the repetitive pattern are selectively attenuated in the bright-field image by inserting a spatial filter at a Fourier transform plane between the two lenses. The resulting image accentuates irregularities on the surface of the integrated circuit, such as may result from contaminating particles or from defects in the pattern.
In this system, however, the wafer is illuminated through the Fourier transform lens. Thus, light scattered and reflected by the lens is added to the image, increasing the level of background illumination. In addition, the relatively strong zero-order reflection from the wafer also passes through the Fourier transform lens producing additional background illumination. To effectively block the illuminated pattern, the Fourier spatial filter used in this system is optically dense. This reduces the amount of light passing through the filter. The combination of all of these effects reduces the sensitivity of the defect detection system.
An example of the other type of defect detection system is given in U.S. Pat. No. 4,772,126 entitled PARTICLE DETECTION METHOD AND APPARATUS, which is hereby incorporated by reference for its teachings on optics and the inspection of semiconductor wafers.
In the system described by this patent, a semiconductor wafer is illuminated at a grazing angle of incidence, between 0.degree. and 5.degree. of the wafer surface. The illuminating beam is oriented to strike the wafer at an angle of approximately 45.degree. with respect to the lines of the rectangular pattern. In addition, the beam is scanned across the surface of the wafer using a scanning galvanometer. Light reflected at angles approximately normal to the surface of the wafer is collected by a video camera positioned above the wafer.
This is a conventional grazing angle configuration, the beam preferentially illuminates particles which extend above the surface of the wafer. The light reflected by these particles is collected by the camera. Particles which extend a greater distance above the surface receive a greater level of illumination, since the surface of the wafer acts as a mirror for this low angle incident radiation. In addition, this patent suggests the use of a Fourier spatial filter to attenuate spatial frequencies, corresponding to the repetitive pattern, from the image collected by the video camera.
While this system works well for detecting particles above the surface of the wafer, it is not as efficient for detecting smaller particles, which may be imbedded in surface features of the wafer, or errors in the repetitive pattern. Since these features are at or slightly below the surface of the wafer, they are not illuminated by the low angle beam and, so, will not appear in the dark field image.
In addition, since the illuminating beam is scanned across the surface of the wafer, the spatial frequency components oscillate in position at the scanning frequency in the Fourier plane. Consequently, a Fourier filter which blocks these spatial frequencies must be opaque over a larger area than if the beam is not scanned. A filter of this type would necessarily decrease the difference in illumination between small particles and defects on one hand and the background illumination levels on the other hand. This decreased difference results in a reduction in the sensitivity of the device to this type of defect when a Fourier spatial filter is used.